2 Answers
2

I will post a separate answer that applies to duplicate information in language in general. In most normal sentences, there are many parts that could be left out without making the sentence unintelligible, because there are often several clues that point to the same interpretation.

I killed him because he killed my
father.

I killed him; he killed my father.

That the because-clause would indicate the motive for killing "him" is clear even without the word "because", because it can be inferred from context that this must be the motive with reasonable probability. That is why the second sentence is still perfectly intelligible on its own. Any aspect of a sentence can be a clue: word order, case, the choice of one word over another, the addition of adjectives and adverbs, expressing the agent of a verb instead of using a passive form, etc., etc.; and in the context of the sentence or in the real world there are always clues as well.

So is the word "because" merely a burden that should be cut out for efficiency? No. There are two reasons:

Oftentimes, the listener misses some part of a sentence, because of noise, distraction, et cetera. In addition, the speaker could make a small mistake or be a bit unclear. The extra information in these "duplicate" clues is an excellent instrument for the listener to repair these lapses and prevent ambiguity. Especially long sentences with many clauses take a toll on the listener or reader and tax his concentration, in which case he will be happy with extra clues.

Even when the listener does not need to make any repairs to what he hears, it takes less effort to understand a sentence if one has several clues. Conciseness can be taxing. The more "inferrable" words that are stated explicity, the less work the listener has to do.

Some languages have less noun/pronoun inflection than English (Chinese languages, for example), and some languages have more inflection (Latin, for example). The answer to your "why" question is that this is simply how the English language developed. You can trace similarities between English and other Germanic languages, and you can look at the history of the language as influenced by political and social factors over time. For example, the Norman invasion is believed to have caused an overall simplification of English grammar because of the language barrier between the Normans and the English.

Note that not all pronouns have inflected forms. For example, "it" can be both subject and object.

What extra information is communicated by expressing object vs subject?

I can't think of a case right now where you would use the extra information outside of poetry. For example, you can change word order and still retain meaning due to the inflected forms:

Thanks! However, an experiment that we did in Latin springs to mind even more fervently. Our linguistics instructor took a random text from Caesar's De Bello Gallico and changed all the endings of declension and cunjugation to nominatives and infinitives. It was our task to reverse the process. Surprisingly, this was not very difficult. It didn't take too much time, and we got the endings right in every instance, except where there was more than one possibility, or where the case was an obscure exception. The meaning of the text remained perfectly clear without the endings.
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CerberusDec 23 '10 at 15:31

OK I got it. Still not sure if it's worth the pain, but at least it makes sense. Thanks @Cerberus and Mitch.
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MarkDec 23 '10 at 15:59